This invention relates to the field of integrated circuit manufacturing. More particularly the invention relates to fabricating integrated circuit resistors having a desired temperature coefficient of resistance.
Precision resistors are critical components in applications such as analog and mixed signal integrated circuits. Reducing the variation of the resistance values of precision resistors over the operational temperature range is critical to maintaining the stability of an analog or mixed signal circuit. Prior resistors have not provided the desired temperature stability.
What is needed, therefore, is a resistor having a desired variation in resistance over temperature. Also needed is method for fabricating such a resistor without significantly increasing the complexity of the manufacturing process in which it is formed.
The above and other needs are met by a resistor having a desired temperature coefficient of resistance and a total electrical resistance. A first resistor segment has a first temperature coefficient of resistance and a first electrical resistance. A second resistor segment has a second temperature coefficient of resistance and a second electrical resistance. The first resistor segment is electrically connected in series with the second resistor segment, and the total electrical resistance equals a sum of the first electrical resistance and the second electrical resistance. The desired temperature coefficient of resistance is determined at least in part by the first temperature coefficient of resistance and the first electrical resistance of the first resistor and the second temperature coefficient of resistance and the second electrical resistance of the second resistor.
Thus, in this manner the desired temperature coefficient of resistance of the resistor can be tailored to a desired value by selecting the resistance and temperature coefficients of resistance of the first and second resistor segments that are connected in series. The desired temperature coefficient of resistance can selectively be a positive value, a negative value, or a zero value, depending upon the selection of the material and the resulting resistance values and temperature coefficient of resistance values for the first and second resistor segments.
In various preferred embodiments of the resistor, the first segment is an unsilicided polysilicon resistor with a negative temperature coefficient of resistance, and the second segment is a silicided polysilicon layer with a positive temperature coefficient of resistance. The electrical resistance of the first segment is preferably related to the electrical resistance of the second segment according to:                     R        1                    R        2              =          "LeftBracketingBar"                        TCR          2                          TCR          1                    "RightBracketingBar"        ,
where R1 is the first electrical resistance of the first segment, R2 is the second electrical resistance of the second segment, TCR1 is the negative temperature coefficient of resistance of the first segment, and TCR2 is the positive temperature coefficient of resistance of the second segment.
Since the first and second segments of the resistor have complementary temperature coefficients of resistance, one negative and one positive, the variation in the values R1 and R2 over temperature are likewise complementary. The total resistance of the resistor RT is the sum of R1 and R2. Thus, the invention provides a resistor having a total resistance RT, which preferably remains substantially constant over a wide temperature range.